The complexity of the T cell - antigen presenting cell interaction and the current simple biochemical approaches being used to study it requires different approach to understand TCR signaling and its regulation. Common interests, existing collaborations and unique expertise, provide us (Drs. Chakraborty, Groves, Kuriyan, Roose and Weiss) with a unique opportunity to study this complex biological system with more sophisticated and novel approaches. We present here a comprehensive program to understand the specificity and regulation of the interacting molecules and the development of a system to study TCR regulated signaling events on a lipid bilayer system to simulate the events occurring essentially on the two dimensional space of the plasma membrane. Such an approach is likely to yield novel insights into molecular interactions and kinetics not obtainable in complex cellular systems and not mimicked by reactions that occur in solution, where diffusion is not limited to 2 dimensions. Indeed, unanticipated results, with marked increase in catalytic activity compared to solution kinetics, were obtained by Dr. Groves and Kuriyan who applied a bilayer system to study the influence of localizing Ras and SOS proteins together at the surface of a bilayer. Our overall objective is to develop a simple but robust biochemical system and computational model of TCR signaling that helps us understand the critical mechanisms that regulate: (Project #1) tyrosine phosphorylation of the TCR-associated immunoreceptor tyrosine-based activation motifs (ITAMs) and of LAT; and, (Project #2) the activation of Ras downstream of LAT by the guanine nucleotide exchange factors (GEFs) RasGRP and SOS. By studying the specificity, regulation and the activities of the molecules involved in a two-dimensional system we attempt to mimic the surface of the inner leaflet of the plasma membrane. We will start with a minimal simple system and iteratively add complexity. We hope to be able to add spatial complexity and molecular complexity. We will study well-defined and important outputs, and will bring to bear not only biochemical and biophysical measures but also computational tools to characterize this system. We will use modeling to compare simple to more complex systems but also study how these simple systems deviate from those obtainable in solution or in more complex cellular systems.